The present invention relates to spacecraft control systems, and more particularly, to spacecraft control systems that provide for control of thermal shock disturbance and errors caused by other repetitive error sources.
Thermal shock disturbance is an industry-wide problem experienced by low earth orbit spacecraft. When a spacecraft enters an eclipse, abrupt temperature changes cause its solar panels to deform. This causes an exchange in momentum between the body of the spacecraft and the panels which results in short-term pointing errors. The severity of the pointing error depends upon the size of the solar wing and other factors.
Conventional solutions to thermal shock disturbance fall into two general categories. The first relates to the mechanical design of the solar panel. By reducing the bending of the solar panel or the momentum it imparts to the spacecraft, optimizing the mechanical design can help alleviate the problem. Clever mounting of the panel can also significantly reduce the momentum imparted to the spacecraft. These solutions have been explored and they tend to increase the cost of the spacecraft significantly. The second solution relates to the design of the control system. A high bandwidth control system may alleviate the pointing disturbance, but avoiding the flex modes of the structure or the transmission of sensor or actuator noise is difficult. Reducing sensor or actuator noise or altering the flex modes of the structure also tend to be high cost solutions.
Feedforward schemes are a reasonable solution but they are limited by the ability of designers to model and construct the signal that needs to be fed forward. Conventional feedforward schemes for controlling thermal shock disturbance are subject to certain limitations: their ability to estimate the disturbance; and the capability to construct a signal that exactly compensates this disturbance. Repetitive control improves upon these methods by taking advantage of the repeatability of the disturbance and using the information inherent in the error signal over previous cycles.
Regarding control of thermal shock disturbance, prior art solutions concentrate on modeling the thermal shock phenomenon and designing solar panels to minimize the transfer of momentum or constructing feedforward schemes. For example, a two-pulse feedforward scheme based on ground based estimates is planned for use on the HS601-C (Galaxy) spacecraft developed by the assignee of the present invention. This scheme is disclosed in U.S. patent application Ser. No. 08/053,056, filed Apr. 26, 1993, entitled "Spacecraft Disturbance Compensation Using Feedforward Control", and is assigned to the assignee of the present invention.
In addition, a number of papers have been written that address thermal shock. These papers include: Sorano, B. C. and J. H. Green, "Dynamic Modelling of Thermal Shock", IEEE Regional conference on Aerospace Control Systems, Thousand Oaks, Calif. 25-27 May 1993; Dennehy, C., D. Zimbelman, and R. Welch, "Sunrise/Sunset Thermal Shock Disturbance Analysis and Simulation for the Topex Satellite", AIAA 90-0470, 281th Aerospace Sciences Meeting, Reno, Nev. 8-11 January 1990; Zimbelman, D, R. Welch and G. Born, "Optimal Temperature Estimation for Modeling the Thermal Elastic Shock Disturbance Torque", AIAA Journal of Spacecraft Vol 28. No 4, 1991, pp 448-456; Zimbelman, D., "Thermal Elastic Shock and. Its Effect on TOPEX Spacecraft Attitude Control," American Astronomical Society, Washington, D.C. Paper 91-056, Feb. 1991; Zimbelman, D., C. Dennehy, R. Welch, and G. Born. "A Technique for Optimal Temperature Estimation for Modeling Sunrise/Sunset Thermal Snap Disturbance Torque", NASA GSFC, Flight Mechanics/Estimation Theory Symposium, 1990 pp. 431-446; Jasper, P. and S. Neste, "UARS Solar Array Snap", General Electric Space Division, U-1K21-UARS-481, Philadelphia, Pa. July 1986; Bainum, P., N. Hamsath and R. Krishna, "The Dynamics and Control of Large Space Structures after the Onset of Thermal Shock", Acta Astronautica, Vol. 19, No. 1, 1989 pp 1-8; Hamsath, N., P. Bainum, and R. Krishna, "The Development of an Environmental Disturbance Torque Model for Large Space Structures after the Onset of Thermal Shock", AIAA paper 86-2123; "Solar Array Flight Experiment/Dynamic Augmentation Experiment", Lockheed Missiles and Space/Marshall Space Flight Center, Ca. 1985; Treble, F, "Solar Arrays for the Next Generation of Communication Satellites", British Interplanetary Society Journal, Vol. 26, Aug. 1973, pp 449-465; and Sudey, J and J. Schulman, "In Orbit Measurements of LANDSAT-D Thematic Mapper Dynamic Disturbances", Acta Astronautica, Vol. 12, No. 7/8,1985, pp. 485-503.
Regarding repetitive control schemes, prior art solutions have primarily concentrated on its application to robotics, although repetitive control has been used on other applications such as disk drives and a synchrotron. A number of papers have been written that address repetitive control. These papers include: Inoue, T., M. Nakano and S. Iwai, "High accuracy control of a proton synchroton magnet power supply", Proceedings of 8th World Congress of IFAC, Vol. XX, pp. 216-221.1981; Tomizuka, M., T. Tsao and K. Chew, "Discrete-time domain analysis and synthesis of repetitive controllers", American Controls Conference, Atlanta, 1988; Tsai, M, G. Anwar and M. Tomizuka, "Discrete time repetitive control for robot manipulators", Proceedings of 1988 IEEE International Conference on Robotics and Automation, Philadelphia, 1988; Tsao, T., and M. Tomizuka, "Adaptive and repetitive control algorithms for noncircular machining", Proceedings of the 1988 American Control Conference, 1988; Cosner, C., G. Anwar and M. Tomizuka, "Plug in repetitive control for industrial robotic manipulators", Proceedings of 1990 IEEE Conference on Robotics and Automation, Cleveland, 1990.
Consequently, it is an objective of the present invention to provide for space-craft control systems that provide for control of thermal shock disturbance and errors caused by repetitive error sources.